1. Field of the Invention
This invention relates to increased solids lubricant dispersions which are the source of calcium stearate, a lubricant present as a component of paper and paperboard coating compositions.
2. Description of the Prior Art
Calcium stearate, a water insoluble metal soap, is a recognized component of paper coating compositions. It contributes lubricating, leveling and anti-dusting properties to paper coating compositions which contain primarily pigment, adhesive and lubricant in an aqueous system.
Where the lubricant is calcium stearate, a water insoluble soap, it is generally supplied as an aqueous dispersion which can be introduced along with the other components in the preparation of the paper coating. Such dispersion contains at most 50 to 55% by weight solids including calcium stearate and dispersing agent. Attempts to increase calcium stearate content result in formation of a paste. Further, when the calcium stearate dispersion is prepared by the in situ method, at most, 55% by weight solids is obtained.
U.S. Pat. No. 2,425,828--Retzsch et al, Aug. 19, 1947, discloses preparation of dispersions of metal soaps including calcium stearate using polyethylene glycol mono-esters of fatty acids as dispersing agents. In the examples, dispersions of 40% calcium stearate, 10% dispersing agent and 50% water (50% total solids) and disclosed.
French publication No. 73-27943, published Mar. 1, 1974, discloses lubricants for starch base paper coatings which contain a first lubricant such as calcium stearate and a second lubricant such as modified tallow, preferably de-acidified tallow, neutralized tallow and hydrogenated tallow. Less preferred are tallow substituted with lower alkyl groups, particular waxes, polyethylene glycols and saturated fatty alcohols. Moderately preferred are fatty acid esters. Somewhat lesser preferred are polyethylene glycol esters of fatty acids.
Kearns et al, Paper Trade Journal, July 24, 1967 (p. 40 et seq), in Table I refer to a 53% minimum calcium stearate dispersion.
Kelly, Jr. et al in Tappi Vol. 53, No. 10, page 1900 et seq, describe studies of the factors affecting performance of aqueous solutions of polyethylene glycols and their derivatives as lubricants when present in coating formulations.
Copending application Ser. No. 623,471--Hill et al, filed June 22, 1984, describes aqueous lubricant dispersions used in the preparation of paper and paperboard coating compositions. These dispersions have a solids content of about 50% by weight to about 75% by weight comprising calcium stearate, dispersing agent for same, nonionic lubricant and urea.
The art, however, is still seeking improved calcium stearate dispersions of enhanced solids content. The advantages of such are reduced shipping costs since less water is shipped. Further, when added to the coating composition, less water is introduced meaning less energy is required to remove same during the drying operation.